Process for producing a multilayer ceramic circuit substrate

ABSTRACT

A method for producing a multilayer ceramic circuit substrate having therein internal conductor patterns comprising W and/or Mo as a main component and surface conductor patterns comprising Cu as a main component formed onto a surface layer of the multilayer ceramic circuit substrate, wherein an intermediate metal layer comprising 40 to 90 wt. % of W and/or Mo and 10 to 60 wt. % of at least one element selected from the group consisting of Ir, Pt, Ti, and Cr is formed in through-holes of the surface layer and on parts of the surface layer in the vicinity of the through holes on the surface layer, whereby the internal conductor patterns and the surface conductor patterns are electrically connected through the intermediate metal layer. The alumina multilayer ceramic circuit substrate enables an excellent bonding strength and electrical conductivity between the internal conductors and the surface conductors and high precision wiring and miniaturization of an electronic circuit part.

This is a division of Ser. No. 08/362,129, filed Dec. 22, 1994, now U.S.Pat. No. 5,627,344.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multilayer ceramic circuit substrateto be used as an electronic circuit part, a process for producing thesame, and an electrically conductive material to be used in theproduction of the same.

2. Description of the Prior Art

In a conventional multilayer ceramic circuit substrate for use in anintegrated circuit, conductors of a high-melting metal are built in, andthe internal conductors are exposed over through-holes of a surfacelayer of the substrate, where they are connected to surface conductorsmade of a thick film comprising copper as a main component. Theforegoing structure involves the problems of a high contact resistanceand a weak bonding strength.

In view of the above, proposals with a view to preventing such problemshave been made, an example of which is disclosed in Japanese PatentApplication Laid-Open No. 218,693/1991. A specific description thereofwill be made while referring to FIG. 3. After green sheets constitutingelectrically insulating layers 11 and internal conductors 12 arecofired, the exposed portions of the internal conductors 12 overthrough-holes 13 of the surface layer of the substrate are covered witha metallo-organic paste comprising Pd, or the like, which is then firedto form Pt, metallized layers 14, which are then coated and covered witha paste comprising Cu as a main component by printing or the like,followed by firing to form surface conductors 15 in the form of a copperthick film.

The foregoing conventional example is excellent as a multilayer circuitsubstrate, but still has room for further improvement. For example, thefiring step comprise three steps: cofiring of the laminated green sheetshaving the internal conductors therein, firing of the metallized layers,and firing for formation of the surface conductors. Thus, a furtherdecrease in the number of steps is desired. Furthermore, since firingcauses deviations in dimensions, the internal conductors are liable tobe partly in direct contact with the surface conductors to cause afailure in connection unless a wide area around the through holeportions are perfectly covered with the metallized layers. Thus, thewide-area metallized layers are formed in an aspect of safety but failto completely meet the requirements for further miniaturization ofwiring patterns. When chip parts are soldered onto the surfaceconductors, the durability of the through-hole portions deteriorates bythe soldering heat to make them unusable as lands for mountingcomponents. Accordingly, the through-hole portions are covered withprotective materials, outside of which the surface conductors areextended out to solder chip parts onto the extended-out portionsthereof. This also presents a problem in further miniaturization ofwiring patterns.

On the other hand, Japanese Patent Application Laid-Open No.196,696/1991 discloses a multilayer ceramic circuit substrate having anintermediate metallic layer of an alloy of at least one of Ni and Cowith at least one of W and Mo between an internal conductor comprising Wand/or Mo as a main component and a surface conductor comprising Cu as amain component, wherein the intermediate metallic layer is provided forthe purpose of improving the adhesion and electrical conductivitybetween the internal conductor and the surface conductor. Afterlaminated insulating green sheets having the internal conductor patternsare cofired, the intermediate metallic layer is formed byelectroplating, electroless plating, or the like on the exposed outersurface of the internal conductors onto which surface the surfaceconductor is to be bonded. Thereafter, an electrically conductive Cupaste is applied onto the intermediate metallic layer by printing, andthen fired to form the above-mentioned surface conductor. Accordingly,the foregoing procedure inevitably involves three steps: cofiring of thegreen sheets and the internal conductor patterns, formation of theintermediate metallic layer, and formation of the layer of the surfaceconductor, which fails to decrease the number of steps.

SUMMARY OF THE INVENTION

The present invention provides a multilayer ceramic circuit substrate ofa high productivity and high quality which can meet the requirements forfurther miniaturization thereof.

In accordance with a first aspect of the present invention, there isprovided a multilayer ceramic circuit substrate having therein internalconductor patterns comprising W and/or Mo as a main component andsurface conductor patterns comprising Cu as a main component formed ontoa surface layer of one side or both sides of the multilayer ceramiccircuit substrate wherein an intermediate metal layer comprising 40 to90 wt. % of W and/or Mo and 10 to 60 wt. % of at least one elementselected from the group consisting of Ir, Pt, Ti, and Cr is formed inthrough-holes of the surface layer and exposed onto an area of thesurface layer in the vicinity of the through holes, whereby the internalconductor patterns and the surface conductor patterns are electricallyconnected through the intermediate metal layer.

According to the present invention, the quality of the substrate isstabilized by forming in the through holes an intermediate metal layercomprising 40 to 90 wt. % of W and/or Mo and 10 to 60 wt. % of at leastone element selected from the group consisting of Ir, Pt, Ti, and Cr andconnecting the internal conductors to the surface conductors through thethrough holes.

A second aspect of the present invention provides a process forproducing the foregoing multilayer ceramic circuit substrate, theprocess comprising:

(a) providing electrically insulating ceramic green sheets havingthrough holes formed therein;

(b) partially or wholly filling through holes formed in a surface greensheet with an electrically conductive paste comprising 40 to 90 wt. % ofW and/or Mo powder and 10 to 60 wt. % of a powder of at least oneelement selected from the group consisting of Ir, Pt, Ti, and Crmutually mixed together with an organic binder and filling the remainingthrough holes with an electrically conductive paste comprising W and/orMo as a main component;

(c) printing the electrically conductive paste comprising W and/or Mo asa main component so as to form internal conductor patterns onto thesurfaces of the green sheets except for the surface green sheet;

(d) laminating the thus treated green sheets;

(e) cofiring the laminated green sheets;

(f) applying a paste comprising C as a main component to form surfaceconductor patterns; and

(g) firing the surface conductor patterns, said surface conductorpatterns being electrically connected to the internal conductor patternsthrough an intermediate metal layer formed from the conductive pastefilled into the through holes of the surface green sheet.

In this process, the above-mentioned surface green sheet becomes eitherone or both surface layers of the resultant multilayer ceramic circuitsubstrate. In the latter case, the conductive paste for the intermediatelayer are filled into the through holes of both surface green sheets andthe surface conductors are also formed onto the intermediate layersformed on both faces of the substrate.

According to this process, the firing and plating steps of forming themetallized layers in the prior art technology can be dispensed with, andthe laminated green sheets can be cofired with the intermediate metallayer represented by W-Ir.

A third aspect of the present invention is to provide an electricallyconductive material used in the preparation of the above-mentionedmultilayer ceramic circuit substrate, the material comprising 40 to 90wt. % of a W and/or Mo powder and 10 to 60 wt. % of powder of at leastone element selected from the group consisting of Ir, Pt, Ti, and Crmutually mixed together with an organic binder.

The multilayer ceramic circuit substrate of the present invention hasexcellent adhesion between the internal conductors and the surfaceconductors thereof, and hence has an excellent bonding strength andelectrical conductivity. Further, according to the present invention,since precise wiring patterns can be formed, miniaturization of anelectronic circuit part can be achieved. Furthermore, according to theprocess of the present invention, part of the firing steps involved inthe prior art technology can be dispensed with, and the deviation indimensions in the products is so low that high-value-added multilayerceramic circuit substrates can be produced with a high productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an example according to the presentinvention.

FIG. 2 is a diagram illustrating the functions of an essential partaccording to the present invention.

FIG. 3 is an illustration of a conventional example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of the ceramics as the substrate material of the presentinvention include alumina, mullite, alumina nitride, etc.

Further, Ir, Pt, Ti, and Cr used in the intermediate metal layer aremetals capable of suppressing the reducing power of W and/or Mo. Arepresentative example of a combination of the former with W and/or Mois W and Ir and this combination is specifically described hereinafterin which an alumina substrate is used by way of example.

Specifically, Ir is formed into a solid solution and alloyed with W bysintering to decrease the amount of elemental W to thereby suppress thereducing power of W, which acts on a Cu paste during sintering of thesurface-layer conductor, whereby the sintering reaction of the Cu thickfilm can be stabilized to turn it into a dense film having a highstrength and electrical conductivity. Further, a small amount of Irdissolves in a solid solution in Cu in the step of firing the Cu thickfilm to form a firm bonding layer around the interface between the Cuthick film and the intermediate metal layer of W and Ir to therebyimprove the bonding strength of the Cu thick film. When the Ir contentis lower than 10 wt. %, the amount of Ir alloyed with W is small and alarge amount of elemental W remains. Therefore, the reducing power of Wcannot be suppressed well because of the large amount of remainingelemental W. On the other hand, when the Ir content exceeds 60 wt. %, adifference in sintering properties between Ir powder and ceramic greensheets such as alumina is enlarged during cofiring with the green sheetsto deteriorate the reliability of the through holes, while the high Ircontent is unsuitable in an economic aspect. The Ir content is moredesirably 35 to 50 wt. %.

The intermediate metal layer of W and Ir may account for either themajority or part of the through hole in the surface layer. Further, ifnecessary, up to 30 wt. %, in terms of outer percentage, of Al₂ O₃ maybe included with W and Ir in the intermediate metal layer in the case ofthe foregoing alumina substrate. Alternatively, when another ceramicmaterial, such as mullite, aluminum nitride or the like is used as thesubstrate material, a ceramic additive consisting essentially of thesubstrate material may also be incorporated into the intermediate metallayer in amount of not greater than 30 wt. %, in terms of outerpercentage, similarly to the alumina substrate. These additives lowerthe probability that a difference in thermal shrinkage during cofiringbetween the corresponding substrate material and a paste filled in thethrough hole for forming the intermediate metal layer is otherwiseenlarged, for example, in the case of a large hole diameter of thethrough-hole portion of the substrate to cause deviation of dimensionsdue to warpage of the substrate and the like. When the amount of theceramic additive to be admixed exceeds 30 wt. %, the influence thereofon the electrical conductivity of the intermediate metal layer isunfavorably increased. The amount of the ceramic additive is moredesirably 5 to 20 wt. %. Throughout the specification, the percentagesof the ceramic additive are all represented by outer percentages byweight, i.e., percentages by weight based on the total weight of theother components of the intermediate metal layer, i.e., W and/or Mo andat least one element selected from the group consisting of Ir, Pt, Ti,and Cr, unless otherwise specified.

The multilayer ceramic circuit substrate of the present invention isproduced in accordance with the process as mentioned above. First, greensheets are prepared in a conventional manner, using a ceramic material,such as alumina, mullite or aluminum nitride and through holes areformed in the green sheets by punching or other known methods. Theformed through holes are filled with a conductive filler paste and aconductive paste is printed onto the green sheets, except for thesurface green sheet to be a surface layer of the resultant multilayerceramic substrate, to form internal conductor patterns using knowntechniques. Then, the green sheets are stacked together into a laminatedstructure and cofired. Subsequently, surface conductor patterns areformed onto the surface layer of the laminated structure by applying aCu conductive paste and firing it.

In the above procedure, the inventive intermediate metal layer is formedby filling through holes as connection portions between the internalconductors and the surface-layer conductors with the above-specifiedpaste comprising W and Mo with at least one element selected from thegroup consisting of Ir, Pt, Ti and Cr mutually mixed together with anorganic binder and cofired with the laminated green sheets. Theinventive conductive paste for forming the intermediate metal layer maybe used in the through holes of surface green sheet(s) for either one orboth faces of the multilayer ceramic substrate and, accordingly, thesurface conductor patterns may be formed on either one or both sides ofthe substrate.

In the paste used for the formation of the intermediate metal layer, thefunctions of the metals represented by W and Ir are as describedpreviously and a common example of the organic binder for use inpreparation of the paste is an ethyl cellulose resin, as a maincomponent, dissolved in a solvent. An example of the thick conductorfilm-forming paste comprising Cu as a main component is a commerciallyavailable product under the trade name of Du Pont #9922.

As has been described, in the electrically conductive paste used for theformation of the intermediate metal layer comprising 40 to 90 wt. % ofpowdered W and/or Mo and 10 to 60 wt. % of powder of at least oneelement selected from the group consisting of Ir, Pt, Ti, and Crmutually mixed together with an organic binder, the paste may furtherinclude up to 30 wt. %, in terms of outer percentage, of alumina powder,mullite powder or aluminum nitride powder, etc., corresponding to thesubstrate material.

The above-mentioned electrically conductive material is suitable as amaterial for connecting the internal conductors comprising W and/or Mopowder as a main component to the thick film surface conductorscomprising Cu as a main component. As described above, at least oneelement selected from the group consisting of Ir, Pt, Ti, and Cr is ametal capable of suppressing the reducing power of W and/or Mo. The mostpreferable combination is W-Ir. As for the W-Ir combination, thefunctions thereof will be described with reference to FIGS. 1 and 2 inconjunction with an exemplified case where this W-Ir combination is usedin an alumina substrate. Laminated alumina green sheets constitutingelectrically insulating ceramic layers 1 and having through holes 3including internal conductors 2 over which a material of a W-Irintermediate metal layer 4 is filled, as shown in FIG. 2. The laminatedalumina green sheets are subjected to cofiring to effect solid solutionof Ir into W to thereby form an alloy, while effecting solid solution ofpart of Ir with W in the internal conductor 2 in the interface 6 thereofto secure a firm bond. Subsequently, a paste comprising Cu as a maincomponent for forming a thick film conductor layer 5 is applied aroundthe W-Ir intermediate metal layer 4 in and over the through hole 3 ofthe alumina insulating layer 1, and then fired to effect solid solutionof part of the Ir with Cu in the interface 7 of the intermediate metallayer 4 with the Cu thick film to thereby form a firm bonding layer,whereby the bonding strength of the Cu thick film can be improved.Further, the amount of the component of elemental W having a strongreducing power against Cu is decreased by the Ir as describedhereinbefore to stabilize the sintering reaction of the Cu thick film tothereby form a thick dense surface conductor film having a highstrength.

Although the foregoing description has been made mainly about the W-Ircombination used in a multilayer alumina ceramic substrate, acombination of Mo and Ir as well as other combinations of W and/or Mowith at least one element selected from the group consisting of Pt, Tiand Cr can secure substantially the same effects. Further, when anotherceramic material, such as mullite, aluminum nitride, etc., is used inplace of alumina, similar effect can be obtained.

The following Examples will illustrate the present invention incomparison with Comparative Examples in detail.

FIG. 1 shows an example of a product according to the present invention,which comprises internal conductors 2 disposed inside a multilayeralumina substrate composed of insulating ceramic layers 1, intermediatemetal layers 4 comprising W and/or Mo and at least one element selectedfrom the group consisting of Ir, Pt, Ti, and Cr and filled in thethrough holes 3 of the surface layers of the multilayer substrate, andCu thick film layers 5 formed around the through holes. A chip part M isfinally mounted on the Cu thick film layers thorough a bonding material8 such as solder.

EXAMPLE 1

A tungsten (W) powder of 3 to 4 μmφ in average particle size was admixedwith an Ir powder having substantially the same particle sizedistribution as the W powder, and then mixed with an organic bindercomprising as a main component an ethyl cellulose resin dissolved in asolvent to prepare a paste. The paste was filled into the through-holeportions of a surface alumina green sheet, which becomes the surfacelayer of a multilayered ceramic substrate after firing, by fillingprinting. A W conductive paste was filled into through holes of otheralumina green sheets and then printed in internal conductor patternsonto the green sheets. The thus treated green sheets were stackedtogether into a laminated structure and then cofired. Subsequently, acommercially available Cu paste (#9922 manufactured by Du Pont Company)was applied on the surface layer with the through holes as centers byprinting to form printed layers of 2 mm×2 mm square in size, followed byfiring to produce a product. The bonding strength of the product wasevaluated in terms of initial strength and strength after being kept ata high temperature of 150° C. The measurement method was as follows. Theproduct was immersed in a molten solder bath having a temperature of235±5° C. Lead wires (wire diameter: 0.6 mmφ) for a tensile test weresoldered onto lands of 2 mm×2 mm square in size on the product. Theinitial bonding strength and the bonding strength after being kept at150° C. for 1,000 hours were measured by a peeling tensile test method.

The paste compositions and bonding strengths of the products as well asratings of evaluation are shown in Table 1. Test results of pastecompositions falling outside the range as specified in the presentinvention are also shown as Comparative Example 1 in Table 1.Additionally stated, No. 10 of Comparative Example 1 is concerned withmetallized layers formed from a Pt metallo-organic paste on a W film. InTable 1, the marks "o" and "x" indicate "good" and "no good",respectively.

                                      TABLE 1                                     __________________________________________________________________________              Paste     Bonding Strength                                                    Composition                                                                             (average value)                                                     (wt. %)   kgf/2 mm × 2 mm                                                 alumina     After                                                             (outer                                                                             Initial                                                                              Keeping at                                            No.    W  Ir                                                                              percentage                                                                         Strength                                                                             High. Temp.                                                                          Remarks     Rating                             __________________________________________________________________________    Ex. 1                                                                             1  85 15                                                                              5    4.7    3.7                ∘                          2  65 35                                                                              0    5.4    4.1                ∘                          3  60 40                                                                              15   5.5≦                                                                          4.3    The measurement of the                                                                    ∘                          4  50 50                                                                              0    5.5≦                                                                          4.5    upper limit value was                                                                     ∘                          5  50 50                                                                              5    5.5≦                                                                          4.4    impossible due to breakage                                                                ∘                          6  60 40                                                                              25   5.5≦                                                                          4.4    of a lead wire in the course                                                              ∘                                                     of measurement of the                                                         initial strength.                              Comp.                                                                             7  97 3 5    The    The                x                                  Ex. 1            measurement                                                                          measurement                                                            was impossible                                                                       was impossible                                                         due to too low                                                                       due to too low                                                         strength.                                                                            strength.                                                 8  95 5 0    The    The                x                                                   measurement                                                                          measurement                                                            was impossible                                                                       was impossible                                                         due to too low                                                                       due to too low                                                         strength.                                                                            strength.                                                 9  93 7 5    0.8    The                x                                                          measurement                                                                   was impossible                                                                due to too low                                                                strength.                                                 10*                                                                              100                                                                              --                                                                              --   4.6    1.5    product with Pt metallized                                                                x                                                                 layers formed.                                 __________________________________________________________________________

In both of Example 1 and Comparative Example 1, the area rate of solderwetting through immersion in the molten solder bath was so good as to beproblem-free in a practical aspect.

In every case of using either a paste having a compositional ratio of Wto Ir as shown in Example 1 or a paste further containing alumina powderadded thereto, both the initial bonding strength and the bondingstrength after the test wherein the product was allowed to stand at thehigh temperature were high, securing good results. In ComparativeExample 1 including No. 10 concerned with the mode of forming the Ptmetallized layers, the initial bonding strengths and the bondingstrengths after the test wherein products were allowed to stand at thehigh temperature were all notably low, demonstrating the poorer bondingperformances than in Example 1.

EXAMPLE 2

Substantially the same procedure as in Example 1, except that part of Wused in Example 1 was replaced with Mo, was repeated to carry outbonding strength tests. The compositions of pastes for formingintermediate metal layers are shown together with the results of thebonding strength tests in Table 2. In Table 2, the mark "o" indicates"good".

                  TABLE 2                                                         ______________________________________                                                          Bonding Strength                                                   Paste      (average value)                                                    Composition                                                                              kgf/2 mm × 2 mm                                              (wt. %)    Initial After Keeping                                             No.    W      Mo   Ir   Strength                                                                            at High temp.                                                                          Rating                           ______________________________________                                        Ex. 2 1      35     30   35   5.2   4.0      ∘                          2      20     45   35   5.0   3.8      ∘                    ______________________________________                                    

EXAMPLE 3

Cases where Pt, Ti, or Cr was used instead of Ir used in Example 1 areshown in Table 3. In Table 3, the mark "o" indicates "good".

                  TABLE 3                                                         ______________________________________                                                          Bonding Strength                                                              (average value)                                             Paste Composition kgf/2 mm × 2 mm                                       (wt. %)           Initial After Keeping                                            No.    W      Pt  Ti   Cr  Strength                                                                            at High temp.                                                                          Rating                         ______________________________________                                        Ex. 3                                                                              1      65     35           3.0   2.3      ∘                       2      65              35  2.7   2.0      ∘                       3      65              35  2.8   2.1      ∘                  ______________________________________                                    

As is apparent from the foregoing, since the bonding strengths betweenintermediate metal layers formed in through-hole portions and the Cusurface conductor layers are very high in the multilayer ceramic circuitsubstrate of the present invention as compared with those in theconventional multilayer ceramic circuit substrates, they hardlydeteriorate, not only in the subsequent steps but also during the courseof long-term service of the multilayer ceramic circuit substrate, whichtherefore has an excellent durability. Additionally, the wettability bysolder of the Cu conductor layers just on the through hole portions isalso improved. Further, since the intermediate metal layers of themultilayer ceramic circuit substrate of the present invention, even ifsmall in area, firmly bond the surface conductors thereof to theinternal conductors thereof without direct contact between the two kindsof conductors, the pitches between the through holes can be minimized tominimize the pitches between the surface conductors as well.Accordingly, chips to be mounted between the surface conductors on thesubstrate can be miniaturized to increase the packaging density thereofwhile enabling miniaturization of circuit components.

What is claimed is:
 1. A process for producing a multilayer ceramiccircuit substrate comprising the steps of:(a) providing a plurality ofelectrically insulating ceramic green sheets having through holes formedtherein; (b) filling the through holes provided in a surface ceramicgreen sheet with an electrically conductive paste composition, saidelectrically conductive paste composition comprising a firstelectrically conductive paste containing from 40 to 90 wt. % of at leastone element selected from the group consisting of W and Mo powder, 10 to60 wt. % of a powder of at least one element selected from the groupconsisting of Ir, Pt, Ti and Cr and an organic binder or said firstelectrically conductive paste and a second electrically conductive pastecontaining at least one element selected from the group consisting of Wand Mo powder as a main component, said first electrically conductivepaste being exposed on an outer surface of said surface ceramic greensheet; (c) printing the second electrically conductive paste ontosurfaces of the other ceramic green sheets to form internal conductorpatterns and filling the through holes of the other ceramic green sheetswith the second electrically conductive paste; (d) laminating all of thegreen ceramic sheets together; (e) cofiring the laminated green ceramicsheets; (f) applying a paste containing copper as a main ingredient onthe surface of the surface ceramic green sheet in contact with the firedexposed first electrically conductive paste to form surface conductorpatterns; and (g) firing the surface conductor patterns; said surfaceconductor patterns being electrically connected to the internalconductor patterns through an intermediate metal layer formed from theelectrically conductive paste composition provided in the through holesof the surface green sheet.
 2. The process for producing a multilayerceramic circuit substrate as claimed in claim 1, wherein the firstelectrically conductive paste additionally contains up to 30 wt. % of apowder of the ceramic material used in forming the ceramic green sheets,said wt. % being based on 100 wt. % of the metals contained in the firstelectrically conductive paste.
 3. The process for producing a multilayerceramic circuit substrate as claimed in claim 2, wherein the ceramicpowder is alumina powder, mullite powder or aluminum nitride powder.